A known way to apply coatings to a receiving surface is to use a distributor to form a layer of coating composition that flows off a horizontal lip of the distributor to create a free-falling curtain of coating composition and to pass the receiving surface through this curtain. The coating composition accelerates by gravity in the curtain, and the increased speed of impingement facilitates higher coating speeds without entraining air. Irregularly shaped objects may be passed through a curtain without breaking it. The small gap between the distributor and the receiving surface required by many coating methods incurs practical disadvantages obviated by curtain coating.
A difficulty in curtain coating is maintaining the width of the free-falling curtain between the lip of the distributor and the receiving surface. Without intervention, a curtain narrows with distance from the lip because of surface tension. A vertical solid surface wetted by the edge of the curtain supports surface tension and maintains curtain width between the lip and receiving surface but may introduce difficulties. Such edge guides exert drag on the coating composition so that an edge portion of the curtain impinges on the receiving surface at a lower speed than the main body of the curtain, and the edge of the curtain may fail to coat. Curved menisci at the edge guide alter the flow distribution locally and thereby create coating nonuniformities. Some coating compositions build up on the edge guide over time. Fouling, surface imperfections, or abrupt changes in geometry of the edge guide produce waves in the curtain that alter coating uniformity up to several inches from the edge guide.
For much of the history of curtain coating, problems at edges have been avoided by forming a curtain wider than the receiving surface, as shown in FIG. 1(a). Indeed, a curtain of sufficient width can have unguided edges and still exceed the width of the receiving surface. In most applications, the curtain consists of a single coating composition, and so the portion of the curtain outside the receiving surface can usually be collected and recycled. In simultaneous multilayer coating, recycling is not possible, and sometimes edge curtains of less costly composition are formed contiguous with the main body of the curtain to reduce the cost of wasted materials.
The biggest disadvantage of a curtain that is wider than the receiving surface is that the entire receiving surface must be coated even though complete coverage is unnecessary or inconsistent with the features of the product. Edge portions of the receiving surface may intentionally differ from the main portion so that an applied coating is wasted; for instance, the edges of a continuous receiving surface may be knurled to aid the winding of rolls of support. The coating composition may wet around the edges of the receiving surface and foul subsequent surfaces of contact.
Many ways to prevent coating of portions of the receiving surface are taught in the art. The edges of the support might be folded downward, as shown in FIG. 1(b) and taught in U.S. Pat. No. 4,975,304, but many receiving surfaces do not have the required flexibility, resiliency, and strength. Several ways are taught for shielding portions of the receiving surface. A simple pan, as shown in FIG. 1(c) and taught in U.S. Pat. No. 3,359,941, does not efficiently evacuate the impinging coating composition, and increasing the height of the walls of the pan introduces the problems of edge guides recited previously. Evacuation can be improved by covering the surface of the pan or blade with a fast moving film of low viscosity liquid, as shown in FIG. 1(d) and taught in International Publication Number WO 90/01179, but the intercepted coating composition is wasted. Disclosed edge guides that deflect part of the curtain, as shown in FIG. 1(e) and taught in U.S. Pat. No. 4,559,896, or as shown in FIG. 1(f) and taught in U.S. Pat. No. 4,879,968, suffer from the known problems of edge guides recited previously; the curtain is guided by a surface that greatly exceeds curtain thickness. This art also teaches the step of significantly angling the edge guide in a vertical plane passing through the lip to widen or narrow the curtain and alter the uniformity of the coating near the edge. Such a step is damaging when the objective is coating a layer uniform to its edges.
However, there are edge guides that avoid most or all of the typical problems. Depending upon the demands of the application, these guides involve some or all of the steps of minimizing the wetted surface area of the edge guide to reduce drag; minimizing the thickness of the edge guide to maintain the two surfaces of the curtain substantially planar and thereby avoid the redistribution of coating composition caused by curved menisci; flushing the guide with a liquid substantially lower in viscosity than the coating composition to reduce drag and prevent fouling; and intercepting and efficiently evacuating a small portion of the curtain adjacent to the edge guide immediately above the receiving surface to minimize excess thickness of the edges of the coating without compromising coating latitude. A guide of suitably small surface area and thickness consists of two thin, smooth, parallel, and closely spaced wires under tension lying in a vertical plane perpendicular to the lip (U.S. Pat. No. 4,830,887). An air flow created by a vacuum source, guided and supported by a blade immediately below the edge guide and above the receiving surface, is effective at removing any lubricating liquid and an adjacent, nonuniform portion of the curtain without reintroducing the problems of edge guides (U.S. Pat. No. 5,395,660).
Besides keeping some portions of the receiving surface dry, it is also sometimes desirable to change the widths of the coated areas without replacing the distributor or altering or replacing some of its elements and perhaps without stopping the process. Continuously variable coating width might replace or augment the steering of the receiving surface to obtain precise registration of the coating; poor registration might be inherently unacceptable or require in compensation wider coating and receiving surfaces than otherwise necessary.